Methods to reduce interference between neighboring networks, which share media, in existing wired systems, are limited. In AC power line systems. the physical lines are not dedicated to a single network. The AC power line network, i.e., the physical network, includes all outlets on a low voltage side of a residential or business transformer. As from four to one-hundred or more residential units may be connected to a single transformer, the physical access points, i.e., the outlets, of a power line network include many dwelling units, houses, apartments, etc. Thus, a desired logical network, typically in a single residence, generally occupies only a small portion of the physical network, the primary purpose of which is to provide AC power to the residences connected thereto. In addition, an AC power line system does not provide any shielding to reduce radiated RF emissions, or to prevent interference from radiated RF emissions, as the network configuration and topology are neither controlled nor designed for communications. Thus power line systems, though physically wired, are quite different from typical wired or dedicated line networks.
Because the physical access points, or nodes, are not isolated to a single logical network, power line networks have more similarity to wireless systems than to wired networks. As in a wireless system, a receiver may hear and receive: 1) the transmission from the intended system; or 2) the transmissions from an unintended, logically different system, e.g., a neighbor; or 3) transmissions from both the intended and unintended systems.
Once a desired network has been defined, regardless of whether it is a wired network or a wireless network, the issue of contention for access to the media needs to be resolved. To resolve the problem of contention for the media, some systems, including some power line systems, use a carrier sense multiple access/collision avoidance (CSMA/CA) method, which is an uncoordinated form of time-multiplexing, which does not use specifically assigned time slots, and which is designed to share capacity on a single physical medium. In lightly loaded systems, CSMA/CA is an effective method to share capacity, however, as system load increases, the capacity available for each node is reduced by the presence of the other nodes contending for the same frequencies, time and general location.
For CSMA/CA systems under heavy load, i.e., when there is contention for capacity, the total utilization may be lower than 60%. Systems utilizing token passing may increase the utilization up to approximately 80%, but, because the overhead for token passing is significant when transmitting small packets token passing may result in lesser utilization of the capacity than in CSMA/CA systems, particularly in networks having a large number of nodes.
In addition to poor utilization, there are Quality Of Service (QoS) problems created as a result of the contention for the media, including increased packet delivery delay and increased variability in packet arrival times, also known as “jitter.” The collision and collision back-off mechanism of CSMA/CA results in reduced utilization, increased delay and increased jitter. Other capacity enhancement techniques are known in the prior art:
Wireless Systems: Cellular
There are wireless systems in the prior art, e.g., code division multiple access (CDMA) mobile phone systems, designed to improve the signal-to-noise and interference ratio (SNIR) of each and every link by reducing power of the links between the mobile transceivers and a base station. In mobile phone systems, the power is controlled by a base station that monitors and controls the power of each mobile device. The base station also “knows” the network topography, i.e., the base station knows that it is at the center of its own cell, or sub-net.
Wireless Systems: Ad hoc Networks
The IEEE 802.11 (Wireless LAN) standard does not establish power control criteria except as a means of statically limiting the transmit power of a device to the maximum power output permitted by the applicable regulatory bodies.
The Bluetooth™ standard implements an optional power control criterion which is used by a receiver to request that a transmitter increase or decrease its transmit power to conform with the receiver's “golden receive power,” which is not specified beyond being a received signal strength intensity (RSSI) range of at least 20 dBm and being the “preferred” power of the device. While this optional power control may have an unintended side effect of helping to reduce overall network interference and thereby increase system capacity, the Bluetooth™ power control criteria does not directly address these goals: if a device's preferred power is too high, The Bluetooth™ technique actively works against reduced interference/increased capacity.
Wired Systems: Dedicated Networks
In a typical wired or dedicated line network or system, such as Ethernet, all nodes may reliably receive any transmission because the dedicated lines of the system are designed to conduct the signal and the network configuration and topology may be controlled. Also, because dedicated line networks include methods of shielding the line to reduce radiated emissions, such networks transmit at sufficient power to permit all nodes in the network topology to reliably receive the signal from any transmitting node. Multiple networks are easily isolated from each other. If communication between networks is required, a “bridge” may be made between specific networks, which bridge may limit the communication to those times when cross network communication is desired.
Wired Systems: Power Line
Because power line networks are relatively noisy, and because power line networks do not have controlled and consistent impedances, most power line systems maintain a power level as high as possible in order to maintain the highest data rate and the highest reliability.
Wired Systems: Other
Home Phone Networking Alliance (HomePNA) and other wired systems are being implemented, however, it is not known whether such systems address the power control issues resolved herein.
Power control methods are used to increase capacity in existing cellular wireless systems and to optimize reception in ad hoc wireless networks; however, power controls methods are not known to be used in wired systems. Dedicated-wired systems do not require power control or other capacity increasing methods because the dedicated wired system is isolated from other wired systems. Power line systems must share the same conductors as neighboring structures supplied by the same transformer, therefore, methods to isolate neighboring systems must be developed and implemented.
U.S. Pat. No. 5,987,333 to Sole, granted Nov. 16, 1999, for Communications power control, describes a power control technique for CDMA systems, base stations and mobile units. The approach to power control is iterative and based on received SNIR. The purpose of such power control is to reduce interfering signal levels, which is advantageous to CDMA systems.
U.S. Pat. No. 5,787,352 to Benveniste, granted Jul. 28, 1998, for System and method for management of neighbor-channel interference with power control and directed channel assignment, describes techniques to assign frequencies and control power of receivers on adjacent channels to minimize interference on the desired channel. It requires that a base station be in control of the channel assignment and power control.
U.S. Pat. No. 5,566,165 to Sawahashi, et. al., granted Oct. 15, 1996, for Transmission power control method and a communication system using the same, describes a CDMA system, having a base station and mobile units. Power control bits based on the received SIR, independent of system load or need, are periodically sent by the base station to a mobile unit, which reduces its transmit power accordingly. This manner of power control is useful in a CDMA system to reduce interference to other mobile units on the same frequencies.
U.S. Pat. No. 5,003,619 to Morris, et. al, granted Mar. 26, 1991, for Method and apparatus for adjusting the power of a transmitter, describes a system wherein peer-to-peer power control is used. The method includes sending codes to the transmitter from the receiver representing the RSSI, but does not send the RSSI itself or the noise or interference level. The purpose of the exchange is to reduce transmit power to increase battery life, not to permit operation of other units in the surrounding area on the same frequency.
U.S. Pat. No. 4,025,853 to Addeo, granted May 24, 1977, for Method and apparatus for radio system cochannel interference suppression, describes a system which requires that each base station provide a different tone, modulated on the same frequency, and that a receiver send the tone of the desired base station. There are no power control methods described to reduce co-channel interference.
In light of the foregoing, there is a clear need to improve the capacity, utilization and quality of service of power line networks when multiple systems share the same physical network.